The prior art contains numerous viscometer constructions. The most common form of viscometer is one in which a first rotary member has an outer surface of revolution which rotates closely adjacent to, but out of contact with, an inner surface of revolution on a second rotary member. The space between the two surfaces defines a cavity in which can be placed a liquid of which the rheological properties are to be determined. Means are typically provided for rotating one of the rotary members, while the other rotary member is placed under a resilient restraint which is such that the extent of displacement of the restrained rotary member from its "at rest" position corresponds linearly with the amount of torque to which it is subjected. The measurement of the displacement, taken together with the geometric properties of the cavity in which the liquid is placed and the rotational speed of the rotating member, permits a calculation of the viscosity of the liquid.
Exemplary of this prior art construction is U.S. Pat. Nos. 4,062,225, Murphy et al, issued Dec. 13, 1977, and 4,045,999, Palmer, issued Sept. 6, 1977.
Almost all of these prior art constructions, as exemplified by the patents just listed, require the liquid to be tested at atmospheric pressure. No capability is present to test the liquid at elevated pressure, or at elevated temperatures.
Another approach taken in the prior art is exemplified by U.S. Pat. No. 3,667,286, Kaufman et al, in which a rotating element has a portion floating in an angular trough, and has a central hub floating in a cylindrical trough. A Newtonian fluid is placed in one of the troughs, and the other trough is filled with the liquid to be tested. The means defining one of the troughs is rotated in a first direction, while the means defining the other of the troughs is rotated in the opposite direction, and these rotational speeds are adjusted until the rotary floating member comes to a standstill. Then, the geometry of the troughs and the relative speeds of rotation can be utilized to calculate the rheological properties of the test liquid. This patent also requires testing to take place at atmospheric pressure, and has no capability of testing at elevated pressures or elevated temperatures.
As is well known, drilling mud is utilized in well-drilling operations for a number of basic purposes. One purpose is to cool and lubricate the bit and the string. Another is to carry up to the surface the chips of rock, etc. which are produced as a result of the drilling operation. A third purpose is to "cake" against the sides of the bore-hole and thus block up the tiny interstices in the rock wall through which the aqueous portion of the drilling mud could escape. A fourth is to control the pressure in the well in order to prevent blow out due to a higher pressure in the formation than in the well. A fifth is for control of corrosion of the string and bit. A sixth purpose is to stabilize the well bore chemistry. A seventh is to buoyantly support the drill string.
Due to geophysical heat in the surrounding formation, the temperature of the drilling mud can rise as high as 600.degree. F. or more. The pressure of the drilling mud depends, of course, upon the depth of the bore-hole. For very deep wells, the pressure placed upon the drilling mud at the bottom of the well can be as high as 20,000 psi or more.
The viscosity of a drilling mud is one of the characteristics which determines how efficiently the drilling mud will carry rock chips, etc. up to the surface. Another factor is the Specific Gravity of the drilling mud.
It is very desirable to be able to measure the viscosity of the drilling mud under conditions which approximate those found under operating conditions, namely at elevated temperatures and pressures. Accordingly, it is an object of an aspect of this invention to provide a viscometer structure which can be pressurized to high pressures and yet function reliably. It is an object of a further aspect of this invention to provide a viscometer structure in which at least the portion in contact with the drilling mud (or other fluid to be tested) may be raised to an elevated temperature without interfering with the operation of the device.
An object of another aspect of this invention is to provide a viscometer in which the measurement range can be adjusted either very slightly or by a substantial amount.
In accordance with this invention, the basic viscometer structure comprises means for rotating a first member at an accurately known rate, and a freely turning rotary member having an axis of rotation and having a first portion adjacent the first member but out of contact with it. A first fluid cavity is thus defined between the first member and the first portion, and a fluid is located in the first fluid cavity such that rotation of the first member will place torque on the rotary member thus seeking to induce rotation therein. The first fluid is preferably a Newtonian fluid such as oil, of which the rheological properties are known.
The rotary member further has a second portion moving adjacent a fixed surface but out of contact therewith, thus defining a second fluid cavity between the second portion of the fixed surface into which can be placed the fluid medium, for example the drilling mud, of which the viscosity is to be measured. The apparatus further includes means to measure the rotational speed of the rotary member.
Additionally, this invention provides a method of measuring the viscosity of a fluid medium, such as drilling mud, comprising several steps. Viscous drag means are used for applying a torque-induced rotation to a rotary member, in such a way that the torque on the rotary member can be known. The fluid medium to be tested in placed in a fluid cavity defined between a portion of the rotary member and a fixed surface, and the torque applied to the rotary member through the viscous drag thereon is determined from a prior calibration. The viscosity of the fluid medium is then determined from the geometry of the fluid cavity, the known torque on the rotary member, and the rotational speed of the rotary member, the latter being measured as one of the steps.
Some drilling muds are non-Newtonian (thixotropic), and the present apparatus is adapted for measuring the viscosity of either Newtonian or non-Newtonian liquids.